KVM: selftests: Fix build with "make ARCH=x86_64"

Marcelo reports that kvm selftests fail to build with
"make ARCH=x86_64":

gcc -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99
-fno-stack-protector -fno-PIE -I../../../../tools/include
-I../../../../tools/arch/x86_64/include -I../../../../usr/include/
-Iinclude -Ilib -Iinclude/x86_64 -I.. -c lib/kvm_util.c
-o /var/tmp/20200604202744-bin/lib/kvm_util.o

In file included from lib/kvm_util.c:11:
include/x86_64/processor.h:14:10: fatal error: asm/msr-index.h: No such
file or directory

#include <asm/msr-index.h>
^~~~~~~~~~~~~~~~~
compilation terminated.

"make ARCH=x86", however, works. The problem is that arch specific headers
for x86_64 live in 'tools/arch/x86/include', not in
'tools/arch/x86_64/include'.

Fixes: 66d69e081b52 ("selftests: fix kvm relocatable native/cross builds and installs")
Reported-by: Marcelo Bandeira Condotta <[e-mail]>
Signed-off-by: Vitaly Kuznetsov <[e-mail]>
Message-Id: <[e-mail]>
Signed-off-by: Paolo Bonzini <[e-mail]>

tools/testing/selftests/kvm/Makefile

···
 INSTALL_HDR_PATH = $(top_srcdir)/usr
 LINUX_HDR_PATH = $(INSTALL_HDR_PATH)/include/
 LINUX_TOOL_INCLUDE = $(top_srcdir)/tools/include
+ifeq ($(ARCH),x86_64)
+LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/x86/include
+else
 LINUX_TOOL_ARCH_INCLUDE = $(top_srcdir)/tools/arch/$(ARCH)/include
+endif
 CFLAGS += -Wall -Wstrict-prototypes -Wuninitialized -O2 -g -std=gnu99 \
 	-fno-stack-protector -fno-PIE -I$(LINUX_TOOL_INCLUDE) \
 	-I$(LINUX_TOOL_ARCH_INCLUDE) -I$(LINUX_HDR_PATH) -Iinclude \